Over the years, many devices have been evolved which attempt to generate, control and use high pressure plasmas for various applications. Each of these devices has certain disadvantageous limitations. The embodiments of the present invention overcome many of the disadvantages of existing technologies by utilizing rotation of the containment envelope to establish a nearly perfect, rigid rotor flow of the gases within. The artificial gravity associated with this rotational flow acts to both center and confine a bubble of plasma and hot gases.
In the context of the present patent document, a plasma is a vapor (usually created from a plasma forming fill in gaseous phase) which includes both neutral particles and charged particles, the latter consisting of electrons and ions. The ions, in turn, may be a combination of atomic ions, charged radicals and/or molecular ions, in which the balance among these different species is dependent on temperature, pressure and the nature of the plasma forming fill. By carefully controlling the environmental conditions in and around the plasma, a plasma column can be formed which is physically isolated from the material boundary of the confinement envelope by an intervening layer of neutral gas. Thus, the plasma column may occupy a cylindrical volume that is smaller than the volume enclosed by the containment envelope.
Since the early 60s, high pressure rf (radio frequency) heated discharges have relied on the use of a “swirl gas” to center the plasma columns produced in such electrodeless devices and to prevent hot plasma from contacting the walls of the containment envelope. Because of the circulatory flow of the injected swirl gases, the term “vortex stabilization” is generally used to characterize this technique. The swirl gas method of vortex stabilization has been applied to a broad range of rf devices. See, for example, Boulos, M. I., in “The Inductively Coupled Radio Frequency Plasma,” High Temp. Material Processes, 1, 17 (1997) or Reed, T. B., J. Appl. Phys., 32, 821 (1961) both of which are hereby incorporated by reference. The swirl gas technique makes it possible to establish plasma columns within containment envelopes at gas pressures from a fraction of atmospheric pressure to many times atmospheric pressure. The plasma columns need not be straight. With the aid of vortex stabilization, a high pressure, rf-heated toroidal discharge has been achieved. See, for example, A. Okin et al., in “Generation of Toroidal Plasma—Atmospheric Ar Gas-Insulated Plasma Source with Quartz and Metallic Discharge Tubes,” Proc. Symp on Plasma Science for Materials, ISSN 0919-7621 (1993) which is hereby incorporated by reference.
In addition to electrodeless discharges heated with rf power, the swirl gas method of vortex stabilization has been applied in electrodeless, high pressure torches powered with microwaves. (e.g., U.S. Pat. No. 5,671,045).
Although the plasma columns formed by the swirl gas technique of vortex stabilization are approximately centered within their containment envelopes and stationary in a gross sense, they are not quiescent on the microscopic level. Viscous drag on the stationary wall of the containment envelope results in a sheared flow of the gas which generates turbulence, giving rise to enhanced thermal transport of energy from the hot plasma column to the cooler envelope walls. Such a degradation in the insulating properties of the annular sheath of cooler gas surrounding the plasma column necessitates an increased power to sustain the plasma discharge. Additionally, turbulence causes mixing of the gas within the sheath and between the gas in the plasma column and that in the sheath. As a practical matter, when such a technique of vortex stabilization is used, the turbulence so generated manifests itself as excess noise in the light emission of the plasma column, which can degrade the accuracy of certain spectroscopic measurements. As a further practical matter, the tip of the plasma plume in vortex stabilized torches is not perfectly stable, demonstrating the characteristic of “wandering” or “flickering” with respect to the center axis of the torch. Therefore, such torches are unsuitable for applications in which precise positioning is required.
The concept of rotating a containment envelope has been used in “sulfur bulb” lamp technologies (e.g., U.S. Pat. Nos. 4,902,935 and 5,404,076) to minimize variations in the spherical bulb's surface temperature (that is, to eliminate hot spots caused by locally high electric fields occurring in resonant microwave cavities or in coaxial termination fixtures) and to make the spatial distribution of visible light emission from these bulbs more uniform. The patents referenced above deal only with sealed bulbs powered by microwaves for application to high intensity lighting. To eliminate hot spots, these patents state that the axis of rotation should be oriented in a certain angular range with respect to the electric field direction in the resonant microwave cavity.
Therefore, what is needed is a method and apparatus for producing a stable, high pressure plasma discharge that can be sustained with a minimum of power. Also, there is a need for creating stable plasma columns which are both long and straight. Also, there is a need for methods and apparatus for generating a stable plasma column inside a containment envelope whereby the effects of shear flow-generated turbulence and buoyancy-driven radial convection are substantially reduced in the gas outside the radius of the plasma column. There is also a need for methods and apparatus for forming a plasma torch having a plasma plume, or flame, which maintains a stable position centered on an axis of rotation of a containment envelope.